A most typical home theater is arranged in such a manner that a home theater main unit is connected with a display and speakers, using lines which are not designed for constructing a network. Since the main unit is connected to the speakers using analog-audio transmission lines, many lines and a complicated arrangement thereof are required. In particular, in case where the home theater main unit is placed in front of the user, the distance between the home theater main unit and a speaker (rear speaker) behind the user is long. A long line connecting these devices is troublesome in circumstances such as home.
To solve this problem, all or some speakers form a network based on a wireless/wired network technology, and audio signals are exchanged over the network. In the case of a wired network in which a home theater main unit and speakers are connected with one another by transmission lines for network use, the arrangement of the lines can be made simpler than a case where the home theater main unit is connected to the speakers using transmission lines for analog sound.
For example, a home theater adopting a 5.1 channel audio system requires at least 6 speakers. When a network is not constructed, at least six transmission lines for analog sound are provided between the home theater unit and the speakers. On the other hand, when a wired network such as LAN (Local Area Network) is constructed, an existing network is available. Therefore each of the home theater main unit and the speakers has only one transmission line for the network, which is connected to each port of the network.
In case where audio signals are transmitted over a network, analog audio signals may be modulated by FM (Frequency Modulation) or AM (Amplitude Modulation). However, analog signals may be deteriorated after being simply modulated.
In case where an audio signal is digitized before the transmission, the transmission of the signal can be conducted through a wireless communication medium such as infrared light and radio wave (see Japanese Laid-Open Patent Application No. 2004-320653; published on Nov. 11, 2004, for example). However, in the case of infrared light, transmission cannot be carried out when an obstacle such as a passer exists between a transmitter and a receiver. The efficiency of transmission between AV (Audio/video) devices is deteriorated in this case.
Standards such as Bluetooth® use a 2.4 Ghz bandwidth which is also used by microwave ovens. For this reason, audio signals may not be properly transmitted by Bluetooth®, when a microwave oven is in operation. The quality of sound, i.e. the quality of transmission between AV devices is therefore deteriorated. Considering the quality, infrared and 2.4 Ghz transmissions should be avoided in case of real-time transmission of video and audio.
The above-described case relates to simplification of an arrangement of lines in a home theater, by transmitting only audio signals on a network. On the other hand, with the growing popularity of the Internet, content on websites can be played on information communication terminals PCs, PDAs (Personal Digital Assistants), and mobile phones, which are remote from the websites.
It is also expected to enable home electric appliances to play content on websites on the Internet. In doing so, it is important to assure interoperability between appliances of different manufacturers. An example of a standard for achieving this interoperability is DLNA (Digital Living Network Alliance).
FIG. 8 shows a case where an AV signal is supplied from an AV source device to a television receiver (hereinafter, television), based on DLNA. In the case shown in the figure, the AV source device 1001 and the television 1002 both of which are compliant with DLNA can be connected with each other without any problems, even if these devices are the products of different manufacturers. To expand the versatility of the AV source device 1001, it is preferable to allow not only the television 1002 but also a home theater to receive a DLNA-compliant AV signal from the AV source device 1001.
The AV source device 1001 is constituted by a video player such as a DVD (Digital Versatile Disk)/Blu-ray Disc player, a DVD/Blu-ray disc recorder, and an HDD (Hard Disk Drive) recorder, or a broadcast receiver such as a BS (Broadcasting Satellite)/CS (Communications Satellite) tuner.
As described above, it is important to allow a home theater to play content on a network, and in doing so, it is required to transmit not only audio signals but also video signals over the network.
In the meantime, lip synchronization is required for playing video and audio content. The lip synchronization means to synchronize the moving images on a television with the sound from a speaker. When out-of-synch for lip synchronization is considerable, synchronization between audio and video is not maintained. According to Non-Patent Document 1 (R. Steinmetz, “Human Perception of Jitter and Media Synchronization”, IEEE Journal On Selected Areas In Communications, Vol. 14, No. 1, pp. 61-72, January 1996), the maximum permissible skew for lip synchronization is about 5 ms. In other words, a skew not shorter than 5 ms is perceptible for human eyes. On this account, AV devices are required to take account of lip synchronization.
In case where audio signals and video signals are transmitted over a network, a further delay occurs during the transmission over the network. It is therefore necessary to compensate a delay due to network transmission, in order to design a high-quality AV device connectable to a network.
Also according to Non-Patent Document 1, stereophonic sound requires the skew between sounds from the both speakers to fall within the range of ±11 μs. In case where respective speakers independently use communication devices, the skew among these communication devices must fall within the range of ±11 μs. As Non-Patent Document 1 defines that a skew for stereophonic sound must fall within the range of ±11 μs, skews of sounds among the speakers must also fall within the range of ±11 μs.
Patent Document 2 (Japanese Laid-Open Patent Application No. 2004-282667; published on Oct. 7, 2004) proposes a transmission device for transmitting audio signals and video signals over a network, and this transmission device has lip synchronization capability. The transmission device includes a wireless transmitter, an audio receiver (e.g. speaker), and a video receiver (e.g. television). The wireless transmitter is directly connected to the output of a typical AV device. Audio signals and video signals from the AV device are supplied to an audio signal encoding section and a video signal encoding section of the wireless transmitter, respectively, and subjected to different processes. The audio signals and video signals are then supplied to the audio receiver and the video receiver, respectively.
However, data streams of, for example, MPEG2 typically include both audio and video data. Therefore, DLNA-compliant transmitters and receivers which are currently under development are designed with the assumption that MPEG2 streams include both audio and video data. According to the architecture proposed by Patent Document 2, even if audio signals and video signals are DLNA-compliant, these types of signals are transmitted by different streams. For this reason, the architecture of Patent Document 2 is not directly interoperable with typical network AV devices (hereinafter, AV stream source devices) which transmit audio and video data by the same stream.
The architecture of Patent Document 2 can be indirectly interoperable with the AV stream source devices, because the architecture is connectable with an external AV device. The interoperability is achieved when the input of the AV device connected to the architecture is interoperable with the AV stream source devices. In this case, the AV device receives, from an AV stream source device, a stream including both audio and video data, separates the audio data from the video data, and outputs the audio data and the video data to the architecture, in a separated manner.
In this case, the audio data and video data are supplied from the AV device to the architecture of Patent Document 2, and then the architecture supplies the audio data and the video data to the audio receiver and the video receiver, respectively. Since the audio data and the video data are transmitted twice, bandwidth efficiency is deteriorated.
For example, in the case of MPEG2 in compliance with HDTV (High Definition Television), the transmission rate of a data stream is at least 20 Mbps. In case where the architecture of Patent Document 2 does not convert the transmission rate of the supplied data stream, two transmissions require at least 40 Mbps. As to wireless transmission, transmission at the transmission rate of not less than 40 Mbps is impossible, because the performance on the MAC-SAP (Media Access Control Service Access Point) of a WLAN (Wireless LAN) module compliant with current IEEE (The Institute of Electrical and Electronics Engineers) 802.11a is 35 Mpbs at the maximum.
IEEE802.11n, which is a currently-developed next-generation wireless LAN standard, is expected to achieve the transmission rate of 100 Mbps or higher on the MAC-SAP. However, this rate is achieved only at short range, and the overall transmission rate in the case of, for example, a home network is assumed to be not higher than 50 Mbps. The above-described transmission rate of 40 Mbps is barely achievable in this case, but significantly heavy load on the transmission path influences on other data transmissions.
For example, IEEE802.11n is expected to support plural HDTV transmissions. When the bandwidth for two transmissions are required for one transmission for HDTV, plural HDTV transmissions cannot be achieved. Also, a home network is expected to be used not only for video data transmission but also for e-mail, Internet browsing, telephone using VoIP (Voice Over Internet Protocol), and Internet games. Since it is important to secure bandwidth for these uses, the quality of service is deteriorated, if bandwidth is inefficiently used.
In case where the transmission rate of a stream supplied to the architecture of Patent Document 2 is converted to a higher transmission rate, the above-described bandwidth problem grows worse. On the other hand, if the transmission rate of a stream supplied to the architecture is lowered, the qualities of transmitted sound and images are deteriorated.